A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet (EUV) radiation which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm, for example 6.7 nm or 6.8 nm.
Radiation may be produced using plasma. The plasma may be created, for example, by directing a laser at particles of a suitable material (e.g. tin), or by directing a laser at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a collector such as a mirrored grazing incidence collector, which receives the radiation and focuses the radiation into a beam. Such a radiation source is typically termed a laser produced plasma (LPP) source.
Japanese publication JP2006216783 describes a multilayer film reflecting mirror which can eliminate a need for light exposure and can previously absorb exciting light which involves thermal expansion of the reflecting mirror. According to this publication, an exciting-light reflection preventive film is formed on a quartz substrate, and an exposure-light reflecting film is formed thereon. Exciting light having a long wavelength passes through the exposure-light reflecting film, and reaches the exciting light reflecting preventive film where the exciting light is absorbed.
JP2006216783 describes two particular embodiments to reduce transmission of laser beam light, at a certain oblique angle of incidence (of 13.5 degrees). In a first embodiment, reflectance of 1064 nm excitation light is about 24%, utilizing an antireflection film comprising several layers of Mo, SiO2, and Si. A second example provides 40% reflectance of 266 nm excitation light, utilizing an antireflection film of HfO2, SiO2, and MgF2 layers.